Field
This disclosure relates generally to ion sensing semiconductor devices, and more specifically, to a mechanism for enhancing sensitivity of ion sensing devices made using silicon on insulator techniques.
Related Art
Ion Sensitive Field Effect Transistors (ISFETs) are often used to sense a concentration of a target ion or molecule in an electrolytic solution. In early ISFET designs, the fluid being sensed was often in intimate contact with the gate dielectric, or separated from the gate dielectric by an ion-sensitive membrane. More current ISFET structures are made using CMOS process flows to create relatively low-cost devices with improved manufacturability. A typical CMOS ISFET includes a sense plate and a CMOS transistor, which includes a floating gate that is coupled to the sense plate. As is typical with a CMOS transistor, the gate is electrically isolated from the transistor channel through a gate dielectric.
In order to measure the ionic concentration, the sense plate interacts with the solution, either directly or through an ion-sensitive material formed on the sense plate, where a reaction can take place between the sense plate material and the ions. Dimensions of the sense plate are typically on the order of 100 square microns. The larger the sense plate area, the higher the sensitivity of the device. But the sense plate is a significant contributor to the size of an ISFET device, and thus larger sense plate areas increase the size of the ISFET device.
In addition, during fabrication of a typical CMOS ISFET, once the gate dielectric is formed, it is susceptible to damage, particularly plasma-induced damage from fabrication processes in which plasmas are present (e.g., during formation of metal layers coupled to the ISFET gate). These processes include, for example, plasma etching, plasma deposition, and the like. Even after fabrication, the gate dielectric may be susceptible to damage from electrostatic discharge (ESD) events or other situations in which the breakdown voltage of the gate dielectric is exceeded.
There is therefore a need for improved ISFET devices that provide enhanced sensitivity to ionic concentration without increasing the dimensions of the sense plate, and which also have decreased susceptibility to plasma-induced damage and electrostatic discharge.
The use of the same reference symbols in different drawings indicates identical items unless otherwise noted. The figures are not necessarily drawn to scale.